1. Field of the Invention
The present invention relates to a driving apparatus using an electromechanical transducer such as a piezoelectric element.
2. Description of the Related Art
Conventionally, impact driving system actuators (impact actuators), which are constituted so that a mobile object such as slider is friction-bonded to a rod-type driving member (driving shaft) so as to be mounted thereto and an electromechanical transducer such as a piezoelectric element is fastened to one end of the driving shaft, are known. For example, in a piezoelectric actuator disclosed in U.S. Pat. No. 6,433,459, a predetermined driving voltage is applied to a piezoelectric element so that the piezoelectric element and a driving shaft are displaced into a saw-tooth wave. As a result, a mobile object is moved along the driving shaft in a dynamical friction state by its inertia according to a time difference between extension and contraction of the piezoelectric element.
When the impact actuator is driven, friction (sliding) is generated between the mobile object and the driving shaft at frequency of, for example, several dozens kHz, and thus frictional heat is generated. Since it is required that the driving shaft is made of a light material with high hardness, the driving shaft is composed of fiber-reinforced resin which is reinforced by carbon whisker or the like. When, however, the actuator is driven for a long time in a high-temperature state (under generation of frictional heat), a resin component in the driving shaft occasionally dissolves. When the actuator is stopped in this state and is left unattended, the dissolved resin component is cooled to be hardened, and the friction-bonded portion between the mobile object and the driving shaft is brought into a fastened state or in a nearly fastened state. As a result, the frictional force becomes stronger than frictional force at the time of the normal driving (standard state). In the impact actuator, the frictional force due to the friction-bonding changes according to environmental conditions such as temperature (the frictional heat and the like) (including another conditions such as a difference in posture, a difference in driving posture, deterioration with age and the like). Further, in the impact actuator, when the driving voltage (driving signal) is applied, not only the frictional force but also a force which is generated by the actuator changes according to the environmental conditions.
For example, when the mobile object and the driving shaft are brought into the fastened state or the nearly fastened state as described above, under the environmental condition such that the force generated by the actuator becomes comparatively weak (for example, in a low-temperature state), a necessary operating speed cannot be obtained and thus the driving becomes unstable at the time of next driving (actuating), namely, the actuator is brought into an operation defective state. Further, not only under the environmental condition such that the actuator is in the operation defective state, but also under the environmental condition at the time of normal driving, the actuator is brought into an operation disabled state such that it is not driven at all.